This section provides background information related to the present disclosure which is not necessarily prior art.
In electric drives and motors, the occurrence of heat losses in the motor is practically unavoidable. Electric motors therefore have an upper temperature limit, and whenever this limit is exceeded, the motor may become damaged or may malfunction, e.g. due to a failure of the winding insulation.
Operating failures of a drive unit or of the electric motor itself can result in excessively high temperatures in the electric motor. Such failures can occur when the machine is operating under high loads, such as with sluggishness or e.g. a blockage of the mechanism. Clogged ventilation grids in an electric motor can also result in undesirable increases in temperature. When the motor winding exceeds the permissible temperature, the electric motor becomes damaged, which may result in a failure of the machine as a whole. It is therefore necessary to monitor the winding temperature in order to protect the electric motor against overheating.
Furthermore, when designing the motor, a compromise must be made between overall dimensions and load torque. Particularly if the motor will be operated nearly constantly below its nominal torque with only brief load peaks occurring above this level, it makes economic sense to design the motor based not on the load peaks but on the projected average value, plus a safety margin. To avoid unnecessary oversizing, it is also necessary to ascertain the motor temperature or in any case to monitor the upper temperature limit.
From unexamined application WO 93/23904 A1 a method for monitoring an electric motor for thermal overloading is known, the object of which is to minimize the technical expenditure and the overall dimensions of the overload protector for the electric motor. To accomplish this, while the electric motor is switched on, the motor power loss or a value that is proportional thereto is calculated based on measured motor data and is then integrated. The integration value is compared with a predetermined threshold value, and when the integration value exceeds a limit for the threshold value, the electric motor is shut off. According to this known method, the actual winding temperature can be ascertained only approximately, and environmental factors, which are co-responsible for the temperature behavior of the electric motor, are disregarded.
In other known methods that likewise do not involve a temperature sensor, the current that is drawn by the electric motor is monitored. If the motor current exceeds the permissible continuous current for an extended period of time, the motor will be shut off by means of a motor protection switch, or in the case of adjustable speed three-phase motors by means of the frequency converter.
Further known methods for protecting an electric motor against overheating involve temperature measurement by means of a temperature sensor, which in most cases is housed in the motor winding. With known embodiments of motors that have overmolded stators, for example, thermal circuit breakers are inserted into the winding prior to the overmolding process and the terminal slots are connected to a circuit board. If the temperature near the motor winding, ascertained in this manner, reaches a predetermined maximum permissible threshold value, this will also trigger a shut off of the electric motor. One method of this type is known from unexamined application DE 199 39 997 A1, the stated object of which is to improve the overload protection of the electric motor particularly with respect to its time characteristic. For this purpose, a signal processor is provided, which generates a corrected temperature signal based on the output signal and at least one past output signal from the temperature sensor.
The devices and methods known from the prior art have a number of disadvantages that lead to problems in practical use. When a temperature sensor is used, an adequate insulating layer is required for electric insulation between the sensor and the winding. This insulating layer also acts as a thermal insulator, and therefore there is a difference in temperature between the electrically active part of the winding and the temperature sensor, particularly in cases of overloads that involve high currents. The result is a delayed shut-off of the electric motor when the motor winding is already overheated. This effect is intensified when the electric insulation must meet increased requirements for the protection of persons. Moreover, when a temperature sensor is installed in the winding, the temperature can be detected at only a single location on the winding.
Particularly when a frequency converter is operated at low infeed frequencies, radically different heating of the winding parts can occur. In this case, the hottest point on the winding, where overheating is already occurring, cannot be reliably determined. The capacitive coupling between the temperature sensor and the motor winding is also problematic particularly in the case of frequency-controlled motors, since coupling currents are transmitted via the temperature sensor line.
Furthermore, integrating a thermal detector or sensor into the winding involves high assembly cost and the need for an early shut off of the motor in the event of a fault, due to the significant temperature difference between the affected winding and the circuit board, which in turn leads to load problems with the motor due to insufficient capacity utilization.
It is therefore the object of the present disclosure to overcome the aforementioned disadvantages by providing an assembly and a method for monitoring the temperature of an electric motor which can be implemented cost-efficiently, reduces the overall cost of assembly and also improves the capacity utilization of the motor, and will reliably detect excess temperatures in the windings of an electric motor so that the motor can be shut off in time in the event of or prior to a thermal overload.
This object is attained by a temperature monitoring assembly and by a motor having such a temperature monitoring assembly.
The basic concept of the disclosure is to conduct the generation of heat and therefore the temperature as a relevant measured variable directly from or from the vicinity of the surface of the motor windings to a sensor by means of a heat transfer medium, which is coupled for the purpose of transferring heat. The sensor can preferably be arranged on an integrated circuit board, e.g. a printed circuit board of the motor.